Cross reference is made to the following applications filed concurrently herewith: entitled xe2x80x9cMethod For Improving Oil Recovery Using An Ultrasound Techniquexe2x80x9d, entitled xe2x80x9cA Method For Removing Trapped Impurity Aggregates From A Filterxe2x80x9d, entitled xe2x80x9cMethod For Dispersing Red And White Blood Cellsxe2x80x9d, entitled xe2x80x9cAn Ultrasonic Method For Improving Cleaning And Redispersal Of Saturated Particle Aggregates In Processes Using Liquid Inksxe2x80x9d, entitled xe2x80x9cAn Ultrasonic Method For Speeding The Drying Of Fluid Saturated Images In Processes Using Liquid Inks xe2x80x3, and entitled xe2x80x9cMethod For Manufacturing Processxe2x80x9d.
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Liquid electrostatic developers having chargeable toner particles dispersed in an insulating nonpolar liquid are well known in the art and are used to develop latent electrostatic images. Ideally, such liquid developers should be replenishable in the particular equipment in which they are used. In general, high solids concentration toners are used for replenishment because relatively low concentrations (e.g., in the range of 10 to 15% by weight solids) result in greater liquid build-up in the equipment, which then must be removed and disposed of as hazardous waste. Thus, it is desirable to initially use a toner containing less liquid, and to maintain the working source located within the equipment, thereby minimizing the undesirable accumulation of carrier liquid in the equipment. When toners are present in the liquid developer in more concentrated form, however, they become difficult to redisperse in the carrier. For example, aggregates may be formed. This can cause serious problems in the replenishment of the liquid developer in the equipment being use. It has been known to use high shear forces between two closely spaced cylindrical surfaces to dissociate liquid toner particles as disclosed in U.S. Pat. Nos. 5,004,165; 5,048,762; 5,078,504; and 5,492,788.
In printing applications these aggregation effects result in grainy images, poor coating uniformity, and poor image to image uniformity and image quality. Breaking up aggregates will result in better image quality. Dispersed particles in inks are subject to many effects that lead to coagulation, limiting shelf life. The liquid-based inks (LID, CEP, and any liquid-based dispersion of small particles) tend to coagulate if left on the shelf for long periods before use. Particles tend to settle under the influence of gravity, producing a sludge layer at the bottom of the container. Brownian motion of the particles due to thermal effects tends to bring particles into contact with one another, leading to coagulation and settling. Charge variations, especially in systems having both sign particles, leads to settling of ink particles. Therefore, it is desirable to have a method and apparatus to readily disperse the particles.
It is also desirable to reuse untransferred cake or inks left on photoreceptors or belts in LID copy or printing engines. However, these materials are often hard to redisperse, and may require the addition of chemical surfactants to obtain new dispersions. However, these additives may change the properties of the inks so that reprocessed inks are not the same in printing characteristics as virgin ink. This limits reuse or recycling of materials. Therefore, it is also desirable to have a method for redispersal to eliminate the need for chemical additives.
High solids suspensions offer greater probabilities for particle-particle collisions, leading to greater rates of coagulation. Thus, high percent solids suspensions offer unique challenges for all of the issues discussed above, eg. shelf life, image quality, and reuse problems. Therefore, it is desirable to have a method to enable the use of concentrated suspensions in more applications.
The methods, as discussed above, have been found acceptable for some applications. However, these dispersion apparatus have the disadvantage in that they require high tolerance, costly mechanical parts to create hydrodynamic sheer to disperse the toner.
A need to provide a less expensive and non-mechanical method for dispersing liquid toner still remains.
It has been found that the above disadvantages can be overcome and liquid developer replenished in the equipment by means of a system whereby aggregated toner that is present is reduced in size by applying an ultrasonic signal to the liquid developer to disperse the toner particles in carrier liquid.
Accordingly, this invention provides a method and an apparatus for dispersing aggregates in a fluid medium. The present invention employs an ultrasonic device to efficiently breakup particle agglomerates by driving the ultrasonic signal over a small range of frequencies around the acoustic slow wave frequency of the saturated agglomerate. At this frequency, the fluid vibrates out of phase with the solid and is forced through the pore structure in the agglomerate. This relative fluid motion exerts high viscous stresses at the particle-particle contact points, which leads to fracture of the agglomerate and the redispersion of the individual particles in the fluid medium. The apparatus includes a dispersing vessel containing aggregates of particles in a fluid, a sonic member for applying an ultrasonic signal in said dispersing vessel for separating the aggregates to form dispersed particles in the fluid medium.